I thought for the first question, it was when it's coming down so at 10m/s. Forces which act on the ball.. do you mean when it collides with the floor or when its coming down before it hits the floor. Is the net force just the initial force - the final force? thank you!

With newtons third law, it's that the force exerted on the floor, should be the same magnitude as the force exerted on the ball by the floor in the opposite direction. Does this mean that the ball exerts a force of 28.8N downwards and the floor exerts a force of 28.8N upwards?

Im confused because the ball goes down, then back up? is it just the dowards for the floor exerts back. so 0.08*10/0.05 = 16N?

No, while the ball is "going down" (but already struck the floor) the floor exerts a force to slow the ball to 0 velocity, but then whle the ball is "going upward", the floor exerts a force giving the ball an upward velocity.

Assuming this is an "elastic" collision, the upward speed of the ball after the collision is 10 m/s, the same as the downward speed before. The net change in velocity is 10- (-10)= 20 m/s and so the net change in momentum is (.08)(20)= 16 kg m/s.